A hydrocarbon-producing formation can be accessed by drilling a wellbore to the formation and opening fluid communication between the formation and the bore of the wellbore for the recovery of hydrocarbons therefrom. Typically, a string of casing is installed along the wellbore and it is known in the industry to perforate the casing using a perforating gun for piercing the casing and affecting the formation to establish fluid communication between the formation and the bore of the cased wellbore for production of the hydrocarbons therefrom.
For a variety of pressure-management issues including safety objectives, perforating has traditionally been conducted in balanced or overbalanced conditions where the fluid pressure in the wellbore at the time of perforating the casing has been equal, greater, or far greater, than the pressure in the formation. Under competing objectives, management of the interface of the formation has resulted in attempts to conduct perforation under both static and dynamic underbalanced conditions wherein the pressure in the wellbore is less than that in the formation. It is thought that the underbalanced conditions during perforating result in a surge or flow which causes the perforations and formation to be cleaned of debris and the like as the fluid flow from the formation surges toward the lower pressure wellbore. In some cases underbalanced perforation has been performed by detonating conventional shaped charges to pierce the casing and, at substantially the same time, canisters are opened in the wellbore for creating a void. Creation of the void and the resulting inrush of fluid results in an enhanced and temporary underbalanced condition which causes fluid to surge from the formation to the wellbore, thereby effecting some degree of cleaning of the perforation and the formation.
Alternatively, as taught in U.S. Pat. No. 6,732,798 to Johnson et al., a porous material is pulverized to expose additional volume to receive wellbore fluids and create the void when activated by an explosive device. U.S. Pat. No. 6,173,783 to Abbott-Brown et al. teaches perforating at extreme overbalanced conditions followed by an underbalanced surge to clean the fractures in the formation. The overbalanced condition is created by forming a fluid column in a tubing string which extends down the casing string to the formation, positioning ports in the tubing string downhole from a packer set in the annulus between the tubing string and the casing. Sufficient gas is added to the fluid column so as to achieve a pressure which exceeds the fracture gradient of the formation. Following perforating the casing, the pressure is maintained below the packer and sufficient volumes of gas are removed from the well so that it is in an underbalanced state after which the ports in the tubing are opened to release the pressure below the packer and cause the flow of fluids to surge from the formation into the tubing string. Typically nitrogen or carbon dioxide are used to charge the tubing string.
US published patent application 2005/0247449 to George et al., teaches using shaped charges in a perforating gun to perforate the casing, preferably at overbalanced conditions. Substantially simultaneously, a combustible element such as a propellant or the like is ignited in a combustion chamber in the perforating gun assembly and the products of the combustion of the combustible element cause a sleeve in a surge canister to shift, opening holes in the canister to the wellbore for creating a dynamic underbalanced condition therein.
There is interest in the industry for improved methods of perforation and production of hydrocarbons which take advantage of the safety and other benefits of balanced and overbalanced perforation as well as the advantages of creating even more pronounced underbalanced conditions.